1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus having the zoom lens. In particular, the present invention relates to a zoom lens useful for an image pickup apparatus, such as a digital camera, a video camera, a monitoring camera, a silver-halide film camera, or a television (TV) camera.
2. Related Background Art
As a zoom lens whose total size is small and which is capable of easily increasing the angle of view, a negative lead type zoom lens has been used. A negative lead type zoom lens typically includes a lens unit (lens group) having a negative refractive power located on the object side (front) of the zoom lens. In a known configuration of a negative lead type zoom lens, a four-unit zoom lens has been used. The four-unit zoom lens of the negative type is generally constituted by, in order from the object side to the image side, a lens unit having a negative refractive power, a lens unit having a positive refractive power, a lens unit having a negative refractive power, and a lens unit having a positive refractive power. In operation, the four-unit zoom lens is configured to execute variation of magnification (zooming) by mechanically changing the distances between the lens units.
In order to accommodate recently-developed image sensors that include a large number of pixels per unit area, new image pickup apparatuses, such as still image and video recording cameras, require a zoom lens having a small total size and high optical performance. In addition to that, it is also desirable a zoom lens having a wide angle of view in order to achieve a large imaging region.
In the field of negative lead type zoom lenses, U.S. Pat. No. 7,777,967 discusses a four-unit zoom lens, which has a zoom ratio as high as 5 times and a wide angle of view of an imaging half angle of view of about 40° at the wide-angle end. Japanese Patent Application Laid-Open No. 2003-131130 discusses a small-size four-unit zoom lens, which has a zoom ratio of about 3 times and an imaging half angle of view of about 35° at the wide-angle end and whose first lens unit is constituted by two lenses.
In general, a negative lead type zoom lens is useful in achieving a wide angle of view, but has a remarkably asymmetric lens configuration. Accordingly, in a four-unit zoom lens in which each lens unit moves during zooming, variation of aberration may increase. Therefore, it becomes extremely difficult to achieve a high optical performance for the entire zoom range while achieving a small total size of the zoom lens.
In particular, if the angle of view is increased in a four-unit zoom lens, curvature of field and chromatic aberration may increase. In the zoom lens discussed in U.S. Pat. No. 7,777,967, the first lens unit is constituted by two negative lenses and one positive lens. With this configuration, the zoom lens discussed in U.S. Pat. No. 7,777,967 corrects curvature of field and chromatic aberration of magnification (lateral chromatic aberration). However, because the first lens unit includes three lenses, the thickness of the first lens unit may increase. As a result, the thickness of the zoom lens when the lens barrel is retracted may increase in the direction of the optical axis.
In the zoom lens discussed in Japanese Patent Application Laid-Open No. 2003-131130, the first lens unit is constituted by one negative lens and one positive lens. With this configuration, the zoom lens discussed in Japanese Patent Application Laid-Open No. 2003-131130 reduces the thickness of the zoom lens in the direction of the optical axis when the lens barrel is retracted. However, the zoom lens discussed in Japanese Patent Application Laid-Open No. 2003-131130 does not have a sufficiently wide imaging angle of view due to the imaging half angle of view at the wide-angle end as low as 35°.
In the above-described four-unit zoom lens, it is significant, in order to achieve a zoom lens whose total size is small, which has a wide angle of view, and which has a high optical performance for the entire zoom range, to appropriately set the refractive power (power) of each lens unit, the lens configuration, and an Abbe number (dispersion) of a material used for a lens constituting each lens unit. More specifically, unless the refractive power of the first lens unit having a negative refractive power and the third lens unit having a negative refractive power and the material of lenses included in the first and the third lens units are appropriately set, it becomes difficult to achieve a zoom lens having a wide angle of view and a high optical performance for the entire zoom range.